The luminosity function of the NoSOCS galaxy cluster sample

We present the analysis of the luminosity function of a large sample of galaxy clusters from the Northern Sky Optical Cluster Survey, using latest data from the Sloan Digital Sky Survey. Our global luminosity function (down to M-r less than or similar to -16) does not show the presence of an 'upturn' at faint magnitudes, while we do observe a strong dependence of its shape on both the richness and the clustercentric radius, with a brightening of M* and an increase in the dwarf-to-giant galaxy ratio with the richness, indicating that more massive systems are more efficient in creating/retaining a population of dwarf satellites. This is observed within both physical (0.5R(200)) and fixed (0.5 Mpc) apertures, suggesting that the trend is either due to a global effect, operating at all scales, or due to a local one but operating on even smaller scales. We further observe a decrease in the relative number of dwarf galaxies towards the cluster centre; this is most probably due to tidal collisions or the collisional disruption of the dwarfs since merging processes are inhibited by the high velocity dispersions in cluster cores and, furthermore, we do not observe a strong dependence of the bright end on the environment. We find an indication that the dwarf-to-giant ratio decreases with increasing redshift, within 0.07 <= z < 0.2. We also measure a trend for the stronger suppression of faint galaxies (below M* + 2) with increasing redshift in poor systems, with respect to more massive ones, indicating that the evolutionary stage of less-massive galaxies depends more critically on the environment. Finally, we point out that the luminosity function is far from universal; hence, the uncertainties introduced by the different methods used to build a composite function may partially explain the variety of faint-end slopes reported in the literature, as well as, in some cases, the presence of a faint-end upturn.